1. Field of the Invention
The present invention relates to a method for manufacturing an optical fiber base material, and more particularly relates to a method for manufacturing an optical fiber base material reducing a bend of an optical fiber base material rod after an elongation in an elongation process elongating base material ingot for optical fiber to be a base material rod having a diameter smaller than before.
2. Description of Related Art
In regard to a method of elongating a base material ingot to be a base material rod having a diameter smaller than before to manufacture an optical fiber base material, there is known a method for heating the base material ingot in an electric furnace, measuring an outside diameter of a heating and melting portion by means of a diameter measurement device while softening the ingot, and controlling a withdrawal speed of the base material rod based on the measured value. The base material rod elongated and obtained in this method may have a bend according to a temperature distribution in an elongating furnace and a set state of the base material ingot.
As measures against this, Japanese Patent Application Publication Nos. 1997-030825 and 1997-030827 disclose a method for measuring a misaligned amount between a central axis line of base material ingot set in an elongating apparatus and a shaft center of the elongating apparatus and changing a gripping angle of the base material ingot or moving a position of the base material ingot, so that the central axis line of base material ingot is identical with the shaft center of the elongating apparatus based on the misaligned amount, in order to elongate the base material ingot while modifying the position of the base material ingot. Moreover, Japanese Patent Application Publication 2000-247664 discloses a method for measuring a bending amount in an elongation and adjusting the number of revolutions of both sides of a roller withdrawing a base material rod, in order to reduce a bend.
The method disclosed in Japanese Patent Application Publication Nos. 1997-030825 and 1997-030827 for controlling the position of base material ingot to be a predetermined position in the elongating apparatus has a problem that a bend is caused even if the base material ingot is controlled to the target position, because a portion hanging the base material ingot or a portion withdrawing the elongated base material rod is changed by maintenance such as the exchange of heater with time and a bias occurs in a temperature distribution in a circumferential direction of a heater or a heat insulator or an airflow inside a furnace. Moreover, the method for adjusting the number of revolutions of a roller disclosed in Japanese Patent Application Publication 2000-247664 has a problem that it is not possible to modify a bend to a direction except the direction of the roller.
In this manner, since a base material rod having a bend caused by elongation significantly damages workability when drawing an optical fiber and stability of an optical fiber characteristic, the base material rod having a bend requires a process for previously modifying this bend when drawing an optical fiber. However, since there is a disadvantage that this process significantly reduces productive efficiency, the development of a method for elongating base material ingot such that a base material rod does not have a bend is requested.